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24 GNU make

you use the value of objects in a recipe, the shell may perform wildcard expansion when

the recipe runs. To set objects to the expansion, instead use:

objects := $(wildcard *.o)

See Section 4.3.3 [Wildcard Function], page 24.

4.3.2 Pitfalls of Using Wildcards

Now here is an example of a naive way of using wildcard expansion, that does not do what

you would intend. Suppose you would like to say that the executable ﬁle foo is made from

all the object ﬁles in the directory, and you write this:

objects = *.o

foo : $(objects)

cc -o foo $(CFLAGS) $(objects)

The value of objects is the actual string ‘*.o’. Wildcard expansion happens in the rule

for foo, so that each existing ‘.o’ ﬁle becomes a prerequisite of foo and will be recompiled

if necessary.

But what if you delete all the ‘.o’ ﬁles? When a wildcard matches no ﬁles, it is left as it

is, so then foo will depend on the oddly-named ﬁle *.o. Since no such ﬁle is likely to exist,

make will give you an error saying it cannot ﬁgure out how to make *.o. This is not what

you want!

Actually it is possible to obtain the desired result with wildcard expansion, but you need

more sophisticated techniques, including the wildcard function and string substitution.

These are described in the following section.

Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to separate

directories in pathnames, like so:

c:\foo\bar\baz.c

This is equivalent to the Unix-style c:/foo/bar/baz.c (the c: part is the so-called drive

letter). When make runs on these systems, it supports backslashes as well as the Unix-

style forward slashes in pathnames. However, this support does not include the wildcard

expansion, where backslash is a quote character. Therefore, you must use Unix-style slashes

in these cases.

4.3.3 The Function wildcard

Wildcard expansion happens automatically in rules. But wildcard expansion does not nor-

mally take place when a variable is set, or inside the arguments of a function. If you want

to do wildcard expansion in such places, you need to use the wildcard function, like this:

$(wildcard pattern...)

This string, used anywhere in a makeﬁle, is replaced by a space-separated list of names

of existing ﬁles that match one of the given ﬁle name patterns. If no existing ﬁle name

matches a pattern, then that pattern is omitted from the output of the wildcard function.

Note that this is diﬀerent from how unmatched wildcards behave in rules, where they are

used verbatim rather than ignored (see Section 4.3.2 [Wildcard Pitfall], page 24).

One use of the wildcard function is to get a list of all the C source ﬁles in a directory,

like this:

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 211 212

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Roland Ver. 4.5 Información técnica Pagina 34